A quantitative study of the mechanisms of cellular integration within a single identifiable crustacean photoreceptive interneuron has been proposed. This interneuron transduces light shined on the terminal abdominal ganglion of the crayfish and related decapod crustaceans into a train of spikes which ascend up the ventral nerve chain. In addition, this cell integrates synaptic input derived from hair receptors on the surface of the telson and uropods. In view of these unique properties and the favorability of this ganglion preparation for neurophysiological research, a study of information transfer within the dendritic field of the photoreceptor interneuron will be carried out. The afferent axons of identified tactile receptors will be filled with intracellular staining dyes, in addition to the staining of the interneuron dendrites, in order to correlate the specificity and locus of synaptic input with respect to their electrical properties. Experiments combining morphology and intracellularly recorded activity from the dendrites in the neuropil are the basis of an extensive study of the mechanisms of synaptic integration, including physiological identification of excitatory and inhibitory synaptic loci, regions of active dendritic spiking, sites of spike initiation and the origin of spontaneous pacemaker activity. These experiments have been designed to demonstrate the temporal and spatial sequence of tactile receptor input as required to code for directional senstivity of water movements in the environment of the tail appendages. How this information is transmitted to higher centers and how the bimodal sensory input serves the animal in terms of behavioral significance will be investigated as well. A comparative study of cellular integration and morphology in related species is also proposed.